1. Field of the Invention
The present invention generally relates to a transformer core, an integral part of distribution/transmission transformers used in power systems, and more particularly, to a plurality of core steel laminations of the transformer core and an assembling method of the plurality of core steel laminations.
2. Description of the Conventional Art
A transformer is a static machine having a core and two or more windings wound on the core. Such a transformer transforms power from one circuit to another without change frequency through electromagnetic induction.
The electromagnetic induction produces an electromotive force across a conductor exposed to time-varying magnetic fields. And most transformers are used to increase or decrease the voltages of alternating current in electric power applications.
For large power transformers, the transformer cores are assembled by arranging a plurality of core steel laminations. And each of the plurality of core steel laminations comprises multiple steel sheets having a silicon content of 3 to 4% and a thickness of 0.23 to 0.35 mm.
In general, such a laminated core of a large-capacity transformer has about 1,000 mm thickness or greater thickness than 1,000 mm. It thus requires stacking of several thousands of silicon steel sheets with 0.23 to 0.35 mm thickness. And, for facilitating the stacking of those silicon steel sheets, one or more holes used to be drilled in the each of the silicon steel sheets depending on manufacturing needs.
FIG. 1 illustrates an example of a conventional transformer core 100 under assembly to form a finished transformer core for large power transformers.
Here, a plurality of core steel laminations 110, 120, 130, and 140 are arranged to receive more silicon steel sheets.
For instance, when the laminated core is completely assembled, the core steel lamination 110 can be then a core bottom yoke. And a result of this, the lamination 130 can be a core top yoke, and the laminations 120 and 140 can be a pair of legs that connect the core bottom yoke and the core top yoke.
For building the laminated core 100, those four core steel laminations 110, 120, 130, and 140 assembled in a stack are bound together by various means.
FIG. 1 does not give details of how to assemble the four core steel laminations. But, in FIGS. 2a, and 2b, the steel sheets 211 and 221 have a splice joint such that each sheet's leading ends joined to the other sheet's leading ends.
In FIG. 1, each of every steel sheets forming the core steel laminations 110, 120, 130, and 140 has at least one hole at its surface with a preset size respectively. For example, those holes indicate the regions that the steel sheets to position 150 and 170 on the first core steel lamination 110.
They also keep its lamination in shape while being assembled to form a finished lamination shape. For the similar purpose of the quick stacking, the second core steel lamination 120 consists of steel sheets with a plurality of holes. And, those holes have an average diameter of 20 to 30 mm.
In FIG. 1, a plurality of arrow lines depicted on the steel sheets illustrates an exemplary flows of the magnetic field when current flow the windings (not indicated) wound on the core steel laminations 110, 120, 130, and 140.
Here, due to the holes, the magnetic flux is not fairly uniform throughout an entire surface of the steel sheet. More precisely, the magnetic flux lines adjacent to the holes are more concentrated than the other regions remote from the holes. And such distorted magnetic flux distribution reduces the transformer's electrical performance.
As shown in FIG. 1, those drilled holes occupy the material of the steel sheet such that it reduces the stacking factor of the core. In addition, a burr is formed while punching a stacking hole in each steel sheet.
The burr forms gaps between the stacked steel sheets, thus causing a decrease in the stacking factor of the core. Also, the transformer core with the staking holes produces noise when an alternating current (AC) flows the windings wound on the core. The gaps between each of the stacked steel sheets make the bigger vibration noises.
To solve those technical problems, a method using a hollow container to cover the core steel lamination is proposed for quickly and safely stacking a plurality of one or more than one sheets of core steel materials forming the core steel lamination.
However this method is partially effective because it only eliminates the need of the holes fixing the steel sheet of the lamination. The problem is that making the shape of the hollow container corresponding to a unique shape of the transformer core steel lamination, e.g., a pot-belly shape, is simply a difficult and time and cost consuming task.